1. Field of the Invention
The present invention relates to time base generators and, more particularly, to a digital time base generator and a method for providing a first clock signal and a second clock signal.
2. Description of the Related Art
U.S. Pat. No. 4,521,778 discloses pulse radar ranging systems which provide distance or level measurement based on the direct measurement of the running time of microwave pulses transmitted to and reflected from a target, e. g., the surface of a fill material in a container. The running time for distances of a few meters is in the nanosecond range. Consequently, a special time transformation procedure is required to enable these short time periods to be measured. Here, the microwave pulses are transmitted to the target at a repetition rate or transmit clock frequency which is given by a transmit clock generator. In a signal mixer, the received echo pulses reflected from the target are sampled by cross-correlation with sampling pulses of the same shape as the transmit pulses, but at a sampling clock frequency that is slightly lower than the transmit clock frequency. The cross-correlation and subsequent integration or low-pass filtering leads to an intermediate frequency signal corresponding to the received echo pulses, but time-expanded relative thereto by a factor T1/(T1−T2), where T1 is the transmit pulse repetition period and T2 is the sampling period. The time-expansion allows for amplifying, digitizing and further processing of the echo pulses with conventional techniques.
Providing the transmit clock frequency and the sampling clock frequency requires a time base of very fine resolution, high accuracy, linearity and stability because of their direct relation to the measurement error. The jitter of the time base has to be kept low because, when the time base becomes significant with respect to the period of the carrier signal (e.g., microwave), the intermediate frequency signal undergoes distortions, amplitude fluctuations and/or cancellations.
It is known to generate the time base signals with digital or with analogue circuits. A digital time base generator having two clock signal generators of slightly different frequencies can benefit from the use of crystal oscillators and phase-locked loop (PLL) circuits that allows the achievement of high accuracy and low jitter. However, as the oscillators start up, the phase difference between the clock signals is unpredictable. Therefore, if a detector for a zero phase delay is used, the detector must be able to operate in the picoseconds range. Zero phase detector errors may diminish the merits of the digital solution and the measurement time is increased due to the necessity to add a waiting time for zero phase detection.
On the other hand, an analogue time base generator uses a linear ramp to generate the variable time delay. The thermal drift of the components results in time delay variations over temperature. U.S. Pat. No. 7,446,699 discloses a technique that compensates for such a thermal drift error. However, the analogue signals having slower variation will still produce a larger jitter from the voltage noise than the digital signals having fast transitions.
U.S. Pat. No. 4,943,787 describes a digital time base generator comprising a base clock signal generator that provides a base clock signal at a base frequency, a first frequency multiplier that multiplies the base frequency by a first integer to produce a first auxiliary signal, a first frequency divider that divides the frequency of the first auxiliary signal by the first integer to generate a first clock signal, a second frequency multiplier that multiplies the base frequency by a second integer to produce a second auxiliary signal, and a second frequency divider that divides the frequency of the second auxiliary signal by the second integer to generate a second clock signal. Accordingly, the frequencies of the first and second clock signals are the same and identical to the base frequency. By altering the first or second integer divisor for a given number of cycles, a defined time delay can be established between the first and second clock signal.